This invention relates to a charge pump circuit, and more particularly to a charge pump circuit used for driving a semiconductor non-volatile memory having FAMOS (Floating gate avalanche injection MOS) transistors as respective cells.
In an EPROM or a flash EEPROM, etc., having FAMOS transistors as their cells, a high potential called a program potential, V.sub.PP is externally applied thereto in Order to effect a write operation, i.e., to inject electrons into the floating gate of the cell.
An equivalent circuit at the time of write operation of a typical EPROM is shown in FIG. 1. A program potential V.sub.PP is applied to the drain of a cell transistor N.sub.3 through N-channel transistors N.sub.1 and N.sub.2 as shown. In this example, the transistors N.sub.1 and N.sub.2 are called a "write transistor" and a "select transistor", respectively.
When a high potential is applied to the drain D of the cell transistor N.sub.3 and a program potential V.sub.PP is applied to the control gate CG as shown in FIG. 2, electrons are injected into the floating gate FG by the avalanche injection. Cells to which electrons have been injected undergo an increase in the threshold voltage V.sub.TH. Writing into these cells is, thus, performed. On the other hand, cells to which no electrons are injected undergo no change in the threshold voltage V.sub.TH. Thus, the programming of "0" and "1" is carried out in dependency upon difference of the threshold voltage.
Referring to FIG. 3, there is shown a rise characteristic of the threshold voltage V.sub.TH vs. the program voltage application time to the gate of each cell. This figure shows that, when a potential on the drain is taken as a parameter, the rise characteristic of the threshold voltage varies as indicated by the curves c, b and a in order recited according as the potential on the drain is increased. Accordingly, it is considered that the potential on the drain and the write time T.sub.PW (time until the threshold voltage reaches a predetermined threshold voltage V.sub.THO) are correlative with each other. Namely, it is preferable that the potential on the drain is high if the write time T.sub.PW is desired to be shortened.
To realize this, the following measure is taken: Since the write operation is conducted through the N-channel transistors N.sub.1 and N.sub.2 as previously shown in FIG. 1, gate control potentials V.sub.PG of these transistors N.sub.1 and N.sub.2 as previously shown in FIG. 1, a gate control potential V.sub.PG for these transistors N.sub.1 and N.sub.2 is set to a value higher than the program potential V.sub.PP, thus preventing voltage drops on these N-channel transistors N.sub.1 and N.sub.2. To obtain a high potential of the gate control potential V.sub.PG, a charge pump circuit is used. Such a technology is disclosed in, e.g., J. PATHAK et al., "A 19-ns 250 mW CMOS Erasable Programmable Logic Device", IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-21, No. 5, OCTOBER, 1986.
A conventional circuit and its equivalent are shown in FIGS. 4 and 5, respectively. In this example, N-channel MOS transistors N.sub.6 and N.sub.7 equivalently serve as diodes as shown in FIG. 5, respectively. These transistors serve to block a reverse-current and hold a stepped-up voltage.
As seen from FIG. 4, an output potential (initial potential) V.sub.OUT (0) at the time of beginning of the step-up operation in this charge pump circuit is expressed as follows: EQU V.sub.OUT (0)=V.sub.PP -V.sub.THN4 -V.sub.THN6 -V.sub.THN7 ( 1)
where V.sub.THN4, V.sub.THN6 and V.sub.THN7 represent threshold voltages of the transistors N.sub.4, N.sub.6 and N.sub.7, respectively. This output potential is considerably lower than the applied program potential V.sub.PP. For example, when it is assumed that V.sub.PP =12.5 V and V.sub.THN1 =V.sub.THN3 =V.sub.THN =2.5 V, the output potential will become equal to a value expressed below: EQU V.sub.OUT (0)=12.5-2.5.times.3=5 V.
Further, an initial potential V.sub.B (0) on the point B will become equal to a value expressed below: EQU V.sub.B (0)=V.sub.PP -V.sub.THN4 -VTHN6 (2).
Thereafter, when a clock is delivered to a step-up capacitor C, a potential on the point B is stepped up by to a potential of the clock (e.g., a value corresponding to V.sub.PP where the clock is of the V.sub.PP system). As a result, this potential is passed through the transistor N.sub.7 (subjected to a voltage drop corresponding to V.sub.THN7) and appears on the output V.sub.OUT. This results in no voltage drop in the transistor N.sub.5. Thus the program potential appears on the point A as it is and a potential on the point B is expressed as follows EQU V.sub.B =V.sub.PP =V.sub.THN6 ( 3).
This potential is further stepped up by a potential of the clock (e.g., a value corresponding to V.sub.PP) and is subjected to voltage drop corresponding to the threshold voltage V.sub.THN7 of the transistor N.sub.7, with the result that it appears on the output V.sub.OUT. Accordingly, the output potential V.sub.OUT finally obtained is expressed at the maximum as follows: EQU V.sub.OUT =V.sub.PP -V.sub.THN6 +V.sub.CLOCK -V.sub.THN7 ( 4),
wherein V.sub.CLOCK is a potential of the clock (e.g. V.sub.PP).
It is to be noted that a scheme is practically employed to further provide a transistor N.sub.8 for limiter on the output point, thus limiting the output potential V.sub.OUT to about V.sub.PP +.alpha. (.alpha. is a desired step-up value). In addition, a scheme is also employed to deliver an ordinary potential V.sub.DD for read operation through a D (depletion) type transistor N.sub.9 at the time of nonprogramming, thus to stand by readout operation.
The problem with such a conventional charge pump circuit is that the initial value V.sub.OUT of the output potential is lower than the program potential V.sub.PP as given by the equation (1). Inconvenience due to this fact occurs mainly when the load of the step up circuit is large. Namely, since the step up efficiency is poor in the case of a large load, it will take much time for rising of the output as described above. For this reason, write time T.sub.PW may be rather prolonged.